I would like to enforce a constraint that an element must have one and/or
the other of two child elements. That is, the two child elements are both
0..unbounded, but one or the other must be present.

To put it another way, if B is absent, A must be present, but if B is
present, A is optional.

<xs:choice>
	<xs:element name="A" type="xs:string" maxOccurs="unbounded"/>
	<xs:element name="B" type="xs:string" maxOccurs="unbounded"/>
</xs:choice>